The present invention generally relates to polarimetry (measurement on optical activity of specimen, expressed in angle of rotation), and more particularly to an improvement in a sample cell for accommodating a liquid specimen subjected to the polarimetry.
The polarimetry has heretofore been applied to identification, examination on purity, determination, and the like of a solute in a liquid specimen. Specifically, the polarimetry is employed in determining the concentrations of fructose, sucrose, glucose and the like contained in an aqueous solution. In recent years, an application of the polarimetry to an examination of urine sugar value (glucose concentration) or urine protein value (albumin concentration) is also proposed (International Patent Publication No. WO 97/18,470).
An angle of rotation ".alpha." of a solution containing optically active substance is directly proportional to a product of a specific rotatory power "[.alpha.]" of the optically active substance and a concentration "C" thereof. If it is assumed that the length of the light path for the measurement is "L", then the angle ".alpha." is represented by the following equation (1): EQU .alpha.[degree]=L[cm].times.[.alpha.].times.C[kg/dl] (1)
It is therefore possible to derive a concentration of the optically active substance contained in the liquid specimen by measuring the angle of rotation of the liquid specimen.
One of the conventional methods of examining sugar or protein in a urine includes a use of a test paper impregnated with a reagent. The test paper is dipped in the urine and a color reaction thereof is observed by a spectrophotometer or the like. In this method, expendable supplies such as test papers are required.
Glucose and albumin in the urine demonstrate optical activities but the other components in the urine do not demonstrate the optical activity. In view of this point, the above publication proposes derivations of the urine sugar value and urine protein value by the polarimetry on the urine. According to this method, even if the glucose or the albumin contained in the urine is small, it is possible to determine the urine sugar value or urine protein value without using any expendable supplies. In this publication, the rotated angle of the plane of vibration i.e., the angle of rotation, is directly derived by projecting a light having a particular plane of vibration on a specimen to be detected and detecting a plane of vibration of the light transmitted through the specimen by using a rotary analyzer.
An example of the conventional polarimeter is shown in FIG. 15. A light source 81 configured with a sodium lamp, a band-pass filter, a lens, a slit and the like projects a substantially parallel light composed of a sodium D ray having a wavelength of 589 nm. A polarizer 82 transmits only a component that has a specific plane of vibration coincident with a transmission axis thereof, out of the light projected from the light source 81. A sample cell 83 for holding a specimen to be determined is arranged so that the light transmitted through the polarizer 82 can transmit therethrough. An analyzer 84 transmits only a component that has another specific plane of vibration, out of the light transmitted through the sample cell 83. An analyzer rotator 85 is for rotating the transmission axis of the analyzer 84 in a plane perpendicular to the direction of the advance of the light. An photosensor 86 is for detecting the light transmitted through the analyzer 84. The computer 87 controls the analyzer rotator 85 while recording and analyzing an output signal from the photosensor 86.
The principle of this polarimeter for the measurement will be explained as follows. In FIG. 16, the abscissa represents the relative angle ".THETA." formed between the light transmission axis of the polarizer 82 and the light transmission axis of the analyzer 84, and the ordinate represents an intensity "I" of the light that has reached the photosensor 86, i.e., the output signal of the photosensor 86. Herein, the solid line indicates the output signal in the case where the specimen to be determined demonstrates no optical activity. Under this condition, the relationship between ".THETA." and "I" is represented by the following equation (2): EQU I=T.times.I.sub.o .times.(cos .THETA.).sup.2 (2)
where, "T" is transmittance of the specimen, and "I.sub.o " is an intensity of the light incident upon the specimen. Herein, a transmission loss and a reference loss of the sample cell 83 and the analyzer 84 respectively are ignored. As shown, a point where "I" reaches its minimum (hereinafter, to be referred to as "extinction point") appears for every .pi. with the variation in ".THETA.", i.e., the rotation of the analyzer 84.
In a case wherein the specimen demonstrates an optical activity and its angle of rotation is ".alpha.", the intensity "I.sub..alpha. " of the light that reaches the photosensor is represented by the dashed line in FIG. 16. The intensity "I.sub..alpha. " is given by the following equation (3): EQU I.sub..alpha. =T.times.I.sub.o .times.{cos (.THETA.-.alpha.)}.sup.2(3)
As seen from this, the extinction point of the specimen which demonstrates an optical activity displaces by ".alpha." as compared with that of the specimen which does not demonstrate the optical activity. It is therefore possible to measure the angle of rotation by finding the displacement of the extinction point by the computer 87. In the case of such polarimeter, S/N ratio in the output signal of the photosensor 86 is however comparatively inferior and it is difficult to accurately determine the position of the extinction point. As a result, it is difficult to measure the specimen having a small angle of rotation with high accuracy.
For this reason, there has been proposed another polarimeter which makes use of optical Faraday Effect, i.e., a phenomenon that when a light is permitted to transmit through a medium while being applied a magnetic field along the direction of its transmission, the direction of polarization of the light rotates with the advance of the light.
The optical Faraday Effect is represented by the following equation (4): EQU a=V.times.H.times.L (4)
where, "a" represents an angle of rotation of the plane of vibration of the light [minute], "V" is Verdet's constant of the medium [minute/A] and "L" is a distance of transmission [m]. Herein, "V" varies with a medium, wavelength of light or temperature.
As one which utilizes this optical Faraday effect, there is an optical Faraday modulator. The optical Faraday modulator includes, for instance, a rod of flint glass and a solenoid coil configured around the rod. When a current is flown through the solenoid coil for generating a magnetic field inside the rod while permitting a light to transmit through the rod along an axis of the rod, the plane of vibration of the light propagating inside the rod rotates. By controlling the intensity of the current flown through the solenoid coil, it is possible to vary the angle of rotation of the plane of vibration at will.
An example of the polarimeter which employs the optical Faraday modulator is shown in FIG. 17. In this figure, parts and components which are identical with those used in the polarimeter shown in FIG. 15 are tagged with the same reference numerals.
The optical Faraday modulator 88 vibrates the plane of vibration of a light transmitted through a polarizer 82 by a modulation signal from a signal generator 89. A lock-in amplifier 90 is for phase sensitive detection of an output signal from the photosensor 86 with reference to the vibration-modulated signal from the optical Faraday modulator 88.
In FIG. 18, the abscissa and the ordinate represent ".THETA." and the output signal "I" of the photosensor, respectively. Herein, FIG. 18 shows the extinction point and the neighborhood thereof in an enlarged view. When the optical Faraday modulator 88 vibration-modulates the plane of vibration with an amplitude of ".delta." and an angular frequency of ".omega.", "I" is given by the following equation (5): EQU I=T.times.I.sub.o .times.(cos [.THETA.-.alpha.+.delta..times.sin(.omega..times.t)]}.sup.2(5),
where, "t" is time.
".THETA." is given by the following equation (6): EQU .THETA.=.pi./2+.beta.(where, .vertline..beta..vertline.&lt;&lt;1)(6) PA1 a tubular base member having a cavity which pierces through the member and connects a pair of end faces of the base member for accommodating a specimen, and a pair of flanges provided around the end faces; PA1 a pair of light-transmitting windows for sealing a pair of open ends of the cavity; and PA1 a coil configured by winding a wire on the base member between the flanges. PA1 arranging a sample cell which comprises a tubular base member having a cavity which pierces through the base member and connects a pair of end faces of the base member for accommodating a specimen, and a pair of flanges provided around the end faces, a pair of light-transmitting windows for sealing a pair of open ends of the cavity, and a coil configured by winding a wire on the base member between the flanges, while inclining an axis of the cavity; PA1 introducing a liquid specimen to be measured into the cavity; and PA1 projecting a light upon the light-transmitting window along the axis of the cavity.
Substituting this equation (6) into the equation (5) gives the following equation (7): EQU I=T.times.I.sub.o .times.{sin[.beta.-.alpha.+.delta..times.sin(.omega..times.t)]}.sup.2(7)
When it is assumed that the angle of rotation attributable to the specimen and an amplitude of the modulation are small, that is, .vertline..alpha..vertline.&lt;&lt;1, and .delta.&lt;&lt;1, the equation (7) is approximated by the following equation (8): ##EQU1##
This indicates that the output signal "I" of the photosensor contains signal components of angular frequency equals 0 (DC), ".omega." and "2.times..omega.", respectively. This is obvious also from FIG. 18. By the phase sensitive detection of the value "I" with the vibration-modulated signal as a reference signal in the lock-in amplifier, it is possible to pick up the component of the angular frequency ".omega.", i.e., the value "S" shown by the following equation (9): EQU S=T.times.I.sub.o .times.2.times.(.beta.-.alpha.).times..delta.(9)
This "S" equals to zero only when .beta.=.alpha., i.e., at the extinction point. In the process of rotating the analyzer, in other words, sweeping ".beta.", the value of ".beta." is the angle ".alpha." of rotation when "S" becomes zero.
As described previously, by modulating the direction of polarization, it is possible to pick up the signal of the modulated frequency component selectively while separating the signal from noises attributable to an intensity of the light source, a fluctuation in the power source, a radiation and the like, thereby to derive a signal "S" with high S/N ratio. Therefore, the extinction point can be determined accurately by using this value of "S", and hence highly accurate measurement of the angle ".alpha." of rotation is permitted.
A sample cell for accommodating the specimen used in the above-mentioned polarimeter has a pair of transparent light-transmitting windows which permit the light to transmit through the inside thereof. Heretofore, the sample cells have been configured, for instance, in a box made of glass with its top end open. Liquid specimens are introduced into the cells through the top open end by the use of a pipette, a syringe and the like.
The measurement is performed for every sample cells and the replacement of the specimen is also performed for every sample cells. Namely, the measurement is performed after introducing the specimen into the sample cell and arranging the sample cell in an optical system. The specimen is therefore required to be replaced together with the sample cell. Further, for using the sample cell again, it is required to exhaust the specimen from the sample cell taken out from the optical system and to wash the sample cell. As described previously, the conventional polarimetry consumes much man power.
In addition, when the specimen is dropped into the sample cell, bubbles are liable to be produced in the specimen. Therefore, it has a problem that the bubbles existing in the optical path during the measurement deteriorates the accuracy of the measurement.